To provide automobile engines with improved fuel efficiency and higher performance for environmental protection, recently, so-called downsizing which reduces engine displacement by 20-50% is accelerated, and direct-injection engines are combined with turbochargers to increase compression ratios. Improvement in the efficiently of engines inevitably results in higher engine temperatures, which may cause power-decreasing knocking. Accordingly, improvement in the coolability of parts particularly around the valves has become necessary.
As a means for improving the coolability of a valve, Patent Reference 1 discloses a method for producing an engine valve comprising sealing metal sodium (Na) in a hollow portion of a hollow valve stem in an engine valve. With respect to a valve seat, Patent Reference 2 teaches a method for directly buildup-welding a valve seat on a cylinder head of an aluminum (Al) alloy by using high-density heating energy such as laser beams to improve the coolability of a valve, which is called “laser cladding method.” As an alloy for buildup-welding the valve seat, Patent Reference 2 teaches a dispersion-strengthened Cu-based alloy comprising boride and silicide particles of Fe—Ni dispersed in a copper (Cu)-based matrix, Sn and/or Zn being dissolved in Cu-based primary crystals.
The valve temperature during the operation of an engine is about 150° C. lower in the above metal-sodium-filled valve (valve temperature: about 600° C.) than in a solid valve, and the Cu-based alloy valve seat produced by the laser cladding method lowers the temperature of a solid valve by about 50° C. (valve temperature: about 700° C.), preventing knocking. However, the metal-sodium-filled engine valves suffer such a high cost that they have not been used widely except some vehicles. The Cu-based alloy valve seats produced by the laser cladding method, which do not contain hard particles, have insufficient wear resistance, suffering seizure by impact wear. Also, the direct buildup-welding on cylinder heads needs the drastic change of cylinder head production lines and large facility investment.
With respect to a valve seat press-fitted into a cylinder head, Patent Reference 3 discloses a two-layer, sintered iron based alloy valve seat comprising a valve-abutting layer (Cu content: 3-20%) and a valve seat body layer (Cu content: 5-25%) formed by using Cu powder or Cu-containing powder for improving thermal conduction, and Patent Reference 4 discloses a sintered Fe-based alloy having hard particles dispersed, which is impregnated with Cu or its alloy.
Further, Patent Reference 5 discloses a sintered Cu-based alloy valve seat, in which hard particles are dispersed in a dispersion-hardened Cu-based alloy having excellent thermal conductivity. Specifically, a starting powder mixture comprising 50-90% by weight of Cu-containing base powder and 10-50% by weight of a powdery Mo-containing alloy additive, the Cu-containing matrix powder being Al2O3-dispersion-hardened Cu powder, and the powdery Mo-containing alloy additive comprising 28-32% by weight of Mo, 9-11% by weight of Cr, and 2.5-3.5% by weight of Si, the balance being Co.
Though Patent Reference 5 teaches that the Al2O3-dispersion-hardened Cu powder can be produced by heat-treating Cu—Al alloy powder formed by atomizing a Cu—Al alloy melt, in an oxidizing atmosphere to selectively oxidize Al, there is actually a limitation to increase the purity of Al2O3-dispersion Cu matrix from an Al-dissolved Cu—Al alloy. Further, the Cu matrix exhibits lower yield strength at higher purity, so that a valve seat is likely detached from a cylinder head as a result of thermal yielding.
Thus, a valve seat capable of suppressing the temperature elevation of a valve on a level not less than those used in expensive metal-Na-filled engine valves, and having excellent wear resistance as well as excellent detachment resistance from a cylinder head, is desired.